1. Technical Field of the Invention
The present invention relates to a liquid-crystal display device, especially to a bend alignment or vertical alignment-mode liquid-crystal display device.
2. Related Art
There has been proposed a liquid-crystal display device (LCD) comprising a bend alignment-mode liquid-crystal cell, in which rod-like liquid-crystal molecules are aligned substantially in opposite directions (symmetrically) in the upper part and the lower part of the liquid-crystal cell (see U.S. Pat. No. 4,583,825 and U.S. Pat. No. 5,410,422). The bend alignment mode is characterized in that, as compared with general liquid-crystal modes such as TN mode and STN mode, the viewing angle is broad and the response speed is high. In addition, since the bend alignment-mode liquid-crystal cell has a self-optical compensatory function, it is referred to as an OCB (optically compensatory bend) liquid-crystal mode.
As a bend alignment-mode liquid-crystal display device improved in terms of viewing angle characteristics, there has been proposed a bend alignment-mode liquid-crystal display device comprising an optical film having an optically-anisotropic layer formed by the use of a discotic compound (see JPA No. hei 9-197397 and WO96/37804).
However, when such an optical film having an optically-anisotropic layer formed by the use of a discotic compound is used for viewing angle compensation in a bend alignment-mode liquid-crystal display device, it has been pointed out that light at a specific wavelength may leak out to cause coloration of display images (see JPA No. hei 11-316378).
Various methods have been proposed for reducing the color shift and for preventing gray scale inversion in a bend alignment-mode liquid-crystal display device that comprises an optical film having an optically-anisotropic layer formed by the use of a discotic compound (see Japanese Patent No. 3056997, JPA Nos. 2002-40429 and 2006-243179).
Recently, the use of liquid-crystal display devices has been expanded and, for example, they are being used even in car navigation systems, instrumental panels, game appliances in rear seats, and head-up displays.
Accordingly, in these applications, bend alignment-mode liquid-crystal displays are required to exhibit a broader latitude for gradation display and are also required to exhibit a further improved transmittance.
For improving the transmittance of liquid-crystal display devices, there are known a method of increasing Δnd of a liquid-crystal cell, and a method of increasing Rth of an optical film (see JPA Nos. hei 9-211444 and 2006-243179).
However, in the method of increasing Δnd of a liquid-crystal cell, the optical characteristics of the optical film may be optimized in accordance with the value of Δnd of the liquid-crystal cell.
In the method of increasing Rth of an optical film, when an additive capable of expressing the optical characteristic is added, then the additive may bleed out and the haze may increase. With that, the production latitude may decrease and the method is difficult to realize.
In the above, a bend alignment mode is mainly described, but vertical alignment-mode liquid-crystal display devices have the same problems as above.